It is generally the practice to remove the head of an internal combustion engine from the block of the engine whenever it is desired to replace seals which exist at the outer ends of the valve members, for sealing against oil leakage around the valve members into the cylinder chambers. This is because that it is necessary to disconnect the seating spring from each valve member in order to do any work at the outer end of the valve member. Once the seating spring has been disconnected from the valve member, the valve member is free to drop into the cylinder chamber where it cannot be retrieved without taking the head off from the block.
Some mechanics have bent pieces of metal and have attempted to insert them through the spark plug hole up into a position against the inner end of the valve member, to in this manner prop the valve member into a substantially seated position. This procedure is very difficult to perform and it is most often not effective. Others have tried to do the same thing by use of screwdrivers or other elements.
A somewhat effective way of holding the valve members into a substantially seated position is to screw an inlet tube into the spark plug hole and then connect this tube to a high capacity compressor. A high capacity compressor has to be used because air is continuously leaking out through the cylinder. Thus, the air must be introduced into the cylinder at a fast enough rate so that there is a net outward force existing on the valve member, sufficient to hold it outwardly. Almost all individuals, and many garages, do not have high capacity compressors and so they cannot practice this technique.